1. Field of the Invention
In many applications a load is to be supplied with a current of variable frequency or amplitude. To do this with high efficiency, and to minimize the size and cost of the components needed, the output power stages are preferably operated in a switched mode. One commonly used arrangement uses two power transistors, such as power MOSFET's or IGBT's, connected in series across the DC supply, with the load connected to the junction between the output transistors--the so-called half-bridge circuit.
Applications of such circuits include electronic ballasts for high intensity gas discharge (HID) lamps, switched mode power supplies having high switching frequencies, and motor drives for electronically commutated DC and AC motors. A commonly applied control method for such integrated circuits (ICs) is pulse width modulation (PWM) as the control signals to achieve current or power regulation, which in turn determines the arc current, motor torque, or the like.
A major problem encountered in designing such half-bridge circuits is the drive for the upper one of the two power devices. This upper device is a high side switch referenced to the output of the half-bridge. This output is a voltage which is varied between approximately zero (0) volts and the DC supply line (rail) voltage; this supply or "rail" can be as high as 500 V for 230 volt power lines.
To minimize the size of components, and to keep any noise produced by an inverter or switched mode amplifier from becoming audible, the half-bridge output circuit is frequently operated at a frequency above 20 kHz so that no audible noise is generated by the circuit.
A drive circuit for a switching half-bridge output should satisfy many requirements, including the following:
Breakdown voltage of at least 500 V PA0 Sufficient gate drive current for fast switching of the power transistors, thus keeping switching losses low PA0 Gate drive current limited to minimize electromagnetic interference PA0 Reliable prevention of simultaneous conduction by the two power devices, to prevent short-circuiting of the DC supply (shoot-through protection) PA0 Direct addressability by one output signal of a microcontroller for each half-bridge leg; that is, input signal levels of 5 to 15 V, without any external delay circuit PA0 Toleration of output voltage slew rates up to 5 to 10 V/ns during switching, particularly with inductive loads PA0 Switching frequency response to 500 kHz PA0 Low power consumption.
2. Description of the Prior Art
Known discrete drive circuits fall into two categories: fast, but relatively inefficient, thus dissipating undesirably large amounts of power; or efficient but slow.
European Patent Application 0 264 614 teaches that resistors should be inserted between each of the output transistors and the common node to which the load is connected, or alternatively the leading edge of the turn-on input signal should be delayed sufficiently to ensure that the other transistor has turned fully off.
A relatively complex high voltage integrated circuit providing shoot-through protection is described in "An HVIC MOSFET/IGT Drive for Half-Bridge Topologies" HFPC May 1988 Proceedings, pp. 237-245.
U.S. Pat. No. 4,740,717 teaches use of a hysteresis circuit in a driver for a half-bridge power stage, to prevent undesired changes in state of the output device due to noise produced in the integrated circuit.
Other techniques for improving shoot-through protection include markedly different structures. Pulse transformers have been used successfully, but are expensive. Optical couplers provide good isolation to the upper device of a full wave output stage, but are often slow and not very reliable; and they consume considerable power for the internal light emitting diodes.